1. Field of the Invention
The present invention relates to a communication system that sends and receives data among apparatuses which store data obtained from a plurality of communication apparatuses and that synchronizes contents of data stored by the apparatuses. Especially, the present invention relates to a sending apparatus, sending method and a communication apparatus utilizing such a sending apparatus that can improve efficiency of data sending operation and can reduce a duty for data communication.
2. Description of Related Art
It is recently known in several fields to utilize a system having many communication apparatuses, in which each communication apparatus is provided with a certain function and connected with other communication apparatuses in order to perform many operations through data exchange between many communication apparatuses. For example, it is known in a field of local area network (LAN) mounted on a motor vehicle to utilize electronic control units (ECUs) as the communication apparatuses, to make each ECU perform a certain operation and to exchange data between the ECUs, in order to provide many functions to be implemented by a system.
A number and a type are increased of the communication apparatuses connected to a communication medium in a system, proportionally to a specializing degree of the function provided to each communication apparatus, as well as an increasing degree of the functions performed by each communication apparatus. Furthermore, the system is expected to perform much more various functions. Thus, each communication apparatus is required to share data and to interact with other communication apparatuses. Then, it results in an increase of exchanged data amount.
To accommodate the increase of exchanged data amount, communication apparatuses of the system is generally divided into some groups, and then each group is connected to a communication medium, respectively. Thus, a number of communication apparatuses connected to the communication medium is reduced. Therefore, it is possible to reduce a communication amount and to avoid, for example, data collisions. Furthermore, it is thought to focus on data type treated by the groups of communication apparatus, and then to connect each group to each of communication mediums respectively having different communication speed. In such a configuration, a communication apparatus is connected with another communication apparatus, by a gateway apparatus that relays data transmission.
A patent document 1 discloses that: ECUs in a field of LAN mounted on a motor vehicle are divided into some groups; each group is connected to each of communication lines mounted on the motor vehicle; the communication lines are connected to each other by a gateway apparatus; priority information is added to the received and the sent data to recognize data priority; the data priority is recognized through the priority information and the data having high priority is preferentially sent and received when the gateway apparatus performs data transmission between different communication lines, to prevent a transmission delay of data having high priority even when the communication lines take increased duty for data transmission.    [Patent document 1] Japanese Patent Application Laid-Open No. 2005-159568